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CHAPTER 3
Surficial
An integral part of the military engineer’s
mission is the location and processing of
materials for construction use. Most construction materials are derived from rocks
and soils that occur naturally on or near the
surface of the earth. These materials may be
obtained by developing a quarry or a borrow
pit.
Quarries are sites where open excavations
are made into rock masses by drilling, cutting, or blasting for the purpose of producing
construction aggregate. These operations require extensive time, manpower, and
machinery. Borrow pits are sites where unconsolidated material has been deposited and
can be removed easily by common earthmoving machinery, generally without
blasting.
Geology
stream to carry the weathered material
decreases; then it deposits the material in a
variety of possible surficial features.
Stream deposits are characteristically
stratified (layered) and composed of particles
within a limited size range. Fluvial deposits
are sorted by size based on the velocity of the
water. When the velocity of the stream falls
below the minimum necessary to carry the
load, deposition occurs beginning with the
heaviest material. In this way, rivers build
gravel and sandbars on the inside of meander
loops and dump fine silts and muds outside
their levees during floods. This creates
deposits of reasonably well-sorted, natural
construction materials.
This chapter covers the processes that form
surficial features which are suitable for
potential borrow pit operations and the types
of construction materials found in these features.
Drainage Patterns
Without the benefit of geologic maps, it is
difficult to determine the type and structure
of the underlying rocks. However, by studying the drainage patterns as they appear on a
topographic map, both the rock structure and
composition may be inferred.
FLUVIAL PROCESS
The main process responsible for the
erosion and subsequent deposition of
weathered material suitable for the development of borrow pits is that of moving water.
When water moves very quickly, as over a
steep gradient, it picks up weathered
material and carries it away. When the
stream slows down (for example, when the
gradient is reduced), the capacity of the
Many drainage patterns exist; however,
the more common patterns are—
Rectangular.
Parallel.
Dendritic.
Trellis.
Radial.
Annular.
Braided.
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Rectangular. This pattern is characterized
by abrupt, nearly 90-degree changes in
stream directions. It is caused by faulting or
jointing of the underlying bedrock. Rectangular drainage patterns are generally
associated with massive igneous and
metamorphic rocks, although they may be
found in any rock type. Rectangular drainage
is a specific type of angular drainage and is
usually a minor pattern associated with a
major type, such as dendritic (see Figure
3-1a). Angular drainage is characterized by
distinct angles of stream juncture.
Parallel. This drainage is characterized by
major streams trending in the same direction,
Parallel streams are indicative of gently dipping beds or uniformly sloping topography.
Extensive, uniformly sloping basalt flows and
young coastal plains exhibit this type of
drainage pattern. On a smaller scale, the
slopes of linear ridges may also reflect this
pattern (see Figure 3-1b).
Dendritic. This is a treelike pattern, composed of branching tributaries to a main
stream. It is characteristic of essentially flatlying and/or relatively homogeneous rocks
(see Figure 3-1c).
Trellis. This is a modified version oft h e
dendritic pattern. Tributaries generally flow
perpendicular to the main streams and join
them at right angles. This pattern is found in
areas where sedimentary or metamorphic
rocks have been folded and the main streams
now follow the strike of the rock (see Figure
3-1d).
Radial. This pattern, in which streams flow
outward from a high central area, is found on
domes, volcanic cones, or round hills (see Figure 3-1e).
Annular. This pattern is usually associated
with radial drainage where sedimentary
rocks are upturned by a dome structure. In
this case, streams circle around a high central
area (see Figure 3-1f).
Braided. A braided stream pattern commonly forms in arid areas during flash
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flooding or from the meltwater of a glacier.
The stream attempts to carry more material
than it is capable of handling. Much of the
gravels and sands are deposited as bars and
islands in the stream bed (see Figure 3-1g and
Figure 3-2, page 3-4), Figure 3-2 shows the
vicinity of Valdez, Alaska. Both the Copper
and Tonsina Rivers are braided streams.
Density
The nature and density of the drainage pattern in an area provides a strong indicator as
to the particle size of the soils that have
developed. Sands and gravels are usually
both porous and permeable. This means that
during periods of precipitation, water percolates down through the sediment. The
density of the drainage and the surface runoff
are minimal due to this good internal
drainage.
Clays and silts are normally porous but not
permeable. Most precipitated water is forced
to run off, creating a fine network of stream
erosion.
Sandstone and shale may exhibit the same
type of drainage pattern. Sandstone, due to
its porosity and permeability, has good internal drainage while shale dots not. Therefore,
the texture or density of the drainage
— pattern
which develops on the sandstone is coarse
while that on shale is fine.
Stream Evolution
The likelihood of finding construction
materials in a particular stream valley can be
characterized by the evolution of that valley.
The evolutionary stages are described as—
Youth.
Maturity.
Old age.
Youth. Youthful stream valleys, which are
located in highland areas, are typified by
steep gradients, high water velocities with
rapids and waterfalls present, downcutting in
stream bottoms resulting in the creation of
V-shaped valleys, and the filling of the entire
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valley floor by the stream (see Figure 3-3a).
Although there is considerable erosion taking
place, there is very little deposition.
Maturity. A mature system has a developed
floodplain and, while the stream no longer
fills the entire valley floor, it meanders to
both edges of the valley. The stream gradient
is medium to low, deposition of materials can
be found, and (when compared with the
youthful stream) there is less downcutting
and more lateral erosion that contributes to
widening the valley (see Figure 3-3b).
Old Age. In an old-age system, the stream
gradient is very gentle, and the water velocity
is low. The river exhibits little downcutting,
and lateral meandering produces an extensive floodplain. Because of the low water
velocity, there is a great amount of deposition.
The river only occupies a small portion of the
floodplain (see Figure 3-3c).
Recognition of the stream evolution stage of
a particular river system is required to
develop sources of construction aggregate.
Rivers in maturity or old age provide the
greatest quantities of aggregate. In youthful
rivers, sources of aggregate are often scarce or
unobtainable due to the steep gradients and
high velocity. Table 3-1, page 3-6, summarizes the characteristics of each stage of
stream evolution, Figure 3-4, page 3-7, shows
an example of the topographic expression of a
youthful stream valley in the vicinity of
Portage, Montana. Figure 3-5, page 3-8,
shows a mature stream valley in the vicinity
of Fort Leavenworth, Kansas. Figure 3-6,
page 3-9, shows an old age stream valley in
the vicinity of Philipp, Mississippi.
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Stream Deposits
Coarse-grained (gravels and sands) and
fine-g-rained (silts and clays) deposits can be
found by map reconnaissance. Certain surficial features are comprised of coarse-grained
materials, others are made up of mediumsized particles, and still others of fine-g-rained
sediments. However, if the source area for a
stream is composed only of fine- grained
materials, then the resulting depositional
features will also contain fine-grained
sediments, regardless of their usual composition.
The following surficial features can be identified by their topographic expressions on
military maps and are likely sources of construction materials.
Surficial Geology 3-6
Point bars.
Channel bars.
Oxbow lakes.
Natural levees.
BackSwamps/floodplains.
Alluvial terraces.
Deltas.
Alluvial fans.
Point Bars. Meandering is the process by
which a stream is gradually deflected from a
straight-line course by slight irregularities.
Most streams that flow in wide, flat-floored
valleys tend to meander (bend). These
streams are alternately cutting and filling
their channels, and as the deflection progresses, the force of the flowing water
concentrates against the channel wall on the
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outside of the curve. This causes erosion on
that wall (a body in motion tends to remain in
motion in the same direction and with the
same velocity until acted on by an external
force) (see Figure 3-7). Consequently, there is
a decrease in velocity and carrying power of
the water on the inside of the curve, and the
gravels and sands are deposited, forming
point bar deposits (see Figure 3-8). Point bar
deposits on many maps will not be apparent
but can be inferred to be at the inside of each
meander loop.
Channel Bars. When a stream passes
through a meander loop, its speed increases
on the outer bank due to the greater volume of
water that is forced to flow on the outside of
the loop. When the stream leaves the
meander and the channel straightens out, the
forces that caused the stream to move faster
are no longer in control and the stream slows
down and deposits materials. These
materials are coarse-grained (gravels and
sands) and are on the opposite bank and
downstream of the point bar. If there is a
series of meander loops, these deposits mayor
may not be present between point bars,
depending on the spacing of the meanders.
However, a channel bar can be expected after
the last meander loop. Figure 3-9, page, 3-12,
shows channel bar deposits, oxbow lakes, and
backswamp/floodplain deposits in the vicinity
of Fort Leavenworth, Kansas. A prominent
channel bar is located north of Stigers Island.
Surficial Geology 3-10
Mud Lake, Burns Lake, and Horseshoe
Lake are oxbow lakes. Backswamps on the
floodplain are represented by swampy ground
symbols.
Oxbow Lakes. During high-water stages, a
stream that normally flows through a
meander loop may cut through the neck of a
point, thus separating the loop. When this
happens, the stream has taken the path of
least resistance and has isolated the bend.
The cutoff meander bend is eventually sealed
from the main stream by fine deposits. The
bend itself then forms an oxbow lake (see Figure 3-10, page 3-13). These deserted loops
may become stagnant lakes or bogs, or the
water may evaporate completely leaving a Ushaped depression in the ground.
Fine-grained deposits (silts and clays) are
normally located in oxbow lakes. An old point
bar deposit can be found on the inside of the U
(see Figure 3-11, page 3-13). In Figure 3-9,
page 3-12. Horseshoe Lake is an example of
the topographic expression of an oxbow lake,
Natural Levees. Stream velocity increases
during flooding as the stream swells within
the confines of its bank to move a greater
volume of water. As the stream moves faster,
it has the ability to carry more material. If
the volume of water becomes so great that the
water cannot stay in the channel, the stream
spills over its banks onto the surrounding
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floodplain, which is a flat expanse of land adjacent to a stream or river. Once the stream
spills over its banks, the water velocity
decreases as the water spreads out to occupy
a larger area. As the velocity decreases, sediment carried by the floodwater is deposited.
The size of this material depends primarily on
the character of the material in the source
area upstream and the velocity of the water in
the stream channel. Generally, gravels and
sands can be found in a natural levee, with
the larger material deposited near the stream
bank and a gradual gradation to smaller sand
particles away from the stream.
Backswamps/Floodplains. After a flood
ends and the stream regresses into its channel, much of the water that spilled over the
banks onto the floodplain is trapped on the
outside of the natural levees. The fine
materials (silts and clays) that are suspended
in this water settle onto the floodplain. Consequently, these areas are often used for
agricultural production. In the lower-lying
areas of the floodplain, a large amount of fines
may accumulate, inhibit drainage, and form
swamplike conditions called a backswamp
(see Figure 3-9, page 3-12).
Alluvial Terraces. A depositing stream
tends to fill its valley with a fair amount of
Surficial Geology 3-14
granular alluvial material. If a change in the
geological situation results in the uplift of a
large area or rejuvenation of the stream, an
increase in the stream velocity by other
means, or a change in the sedimentation and
erosion process, the stream may begin to
erode away the material it had deposited previously. As the eroding stream meanders
about in its new valley, it may leave benchlike
remnants of the preexisting valley fill
material perched against the valley walls as
terraces. This action of renewed downcutting
may occur several times, leaving several terrace levels (see Figure 3-12). These are easily
recognized on a topographic map because
they show up as flat areas with no contour
lines, alternating with steeply sloping regions
with many contour lines. Alluvial terraces
usually occur on one side of the stream but
can be found on both sides. They are a normal
feature of the history of any fluvial valley.
They are usually a good source of sands
and gravels. Figure 3-13 shows alluvial
terraces in the vicinity of Souris River, North
Dakota.
Deltas. When streams carrying sediments
in suspension flow into a body of standing
water, the velocity of the stream is immediately and drastically reduced. As a result, the
sedimentary load begins to settle out of
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suspension, with the heavier particles settling first. If the conditions in the body of
water (sea or lake) are such that these particles are not spread out over a large area by
wave action, or if they are not carried away by
currents, they continue to accumulate at the
mouth of the stream. Large deposits of these
sediments gradually build up to just above
the water level to form deltas (see Figure
3-14). These assume three general forms,
Surficial Geology 3-16
depending mainly on the relative influence of
waves, fluvial processes, and tides. These
forms are—
Arcuate (see Figure 3-15a and b).
Bird’s-foot (see Figure 3-15c,),
Elongate (see Figure 3-15d).
Arcuate deltas are arc- or fan-shaped and
are formed when waves are the primary force
acting on the deposited material. Arcuate
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deltas usually result from deposition by
streams carrying relatively coarse material
(sands and gravels) with some occasional fine
material. Arcuate deltas consisting primarily of coarse material have very good internal
drainage; therefore, they have few minor
channels. On the other hand, an arcuate
delta having a considerable amount of fine
material (silts and clays) mixed with the
coarse material does not have good internal
drainage. In this case, a larger number of
minor channels develop. Generally, arcuate
deltas are considered good sources of sands
and gravels. An example of an arcuate delta
is the Nile Delta in Egypt.
Bird’s-foot deltas are formed in situations
where fluvial processes have a major influence on deposited sediments. Bird’s-foot
deltas resemble a bird’s foot from the air,
hence the name. They are generally composed of fine-g-rained material and have very
poor internal drainage. These deltas are flat
with vegetation, have many small outlets,
and are a good source of fine materials. The
Mississippi Delta is a classic example of this
delta type.
Elongate deltas form where tidal currents
have a major impact on sediment deposition.
They contain only a few distributaries, but
the distributaries that occur are large.
Alluvial Fans. These are the dry land
counterpart of deltas. They are formed by
streams flowing from rough terrain, such as
mountains or steep faults, onto a flat plain.
This type of deposit is found in regions that
have an arid to semiarid type of climate, such
as the western interior, the Basin and Range
Province of the United States, and the desert
mountain areas worldwide. The valleys in
these areas are normally dry much of the
year, with streams resulting only after torrential rainstorms or following the spring
snow melt. The mountains themselves are
devoid of vegetation, and erosion by the
streams is not impeded. These streams rush
down a steep gradient, and when they meet
the valley floor, there is a sudden reduction in
velocity. The sediment load is deposited at
the foot of the rough terrain. This deposit is
in the form of a broad “semitone” with the
apex pointing upstream. Coalescing alluvial
fans consist of a series of fans that have joined
to form one large feature. This is typical in
arid areas. Figure 3-16 depicts alluvial and
coalescing alluvial fans. Alluvial fans may
be readily identified by their topographic
Surficial Geology 3-17
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expressions of concentric half-circular contour lines. Figure 3-17 is a topographic map
showing the Cedar Creek alluvial fan in the
vicinity of Ennis Lake, Montana. This alluvial fan is approximately four miles in
radius. Figure 3-18, page 3-20, shows coalescing alluvial fans in the vicinity of Las Vegas,
Nevada.
move slowly in a plastic state with the ice
churning the soil and rocks beneath it as well
as crushing and plucking rocks from the
ground and incorporating large amounts of
material within the glacier itself. The overall
range of particle size of these materials is
from clays through cobbles and boulders (see
Figure 3-22, page 3-24).
The types of materials found in alluvial
fans are gravels, sands, and fines based on a
1/3 rule. The first 1/3, the area adjacent to
the highland, is primarily composed of
gravels; the middle 1/3 is composed of sands;
and the final 1/3, the area farthest from the
highland, is composed of fines.
Alpine. Alpine or mountain glaciation takes
place in mountainous areas and generally
results in the creation of mainly erosional
forms. Alpine glacial features are very distinctive and easy to recognize. In the past,
glaciers scooped out and widened the valleys
through which they moved, producing valleys
with a U-shaped profile in contrast to the
V-shaped profile produced by fluvial erosion
(see Figure 3-23, page 3-25).
Fluvial features are found throughout the
world and are the primary source of borrow
pit materials for military engineers. Table
3-2, page 3-21, and Figure 3-19, page 3-21,
present a summary of fluvial features. Figure 3-20, page 3-22, shows a generalized
distribution of fluvial surficial features
throughout the world.
GLACIAL PROCESS
Between ten and twenty-five thousand
years ago, much of North America, Europe,
and Northern Asia was covered by glaciers.
Significant ice sheets still cover Greenland
and Antarctica, and lesser ice sheets can be
found at high elevations and latitudes (see
Figure 3-21, page 3-23).
Glaciation produces great changes in the
existing topography by reshaping the land
surface and depositing new surficial features
that may serve as a source of construction aggregate for military engineers.
Types of Glaciation
The glaciation process may be described as
either continental or alpine glaciation.
Continental. Continental glaciation occurs
on a large, regional scale affecting vast areas.
It may be characterized by the occurrence of
more depositional features than erosional
features. Continental glaciers can be of
tremendous thickness and extent. They
Surficial Geology 3-18
Glacial Deposits
Materials deposited by glaciers are frequently differentiated into two types. They
are—
Stratified.
Unstratified.
Stratified. The features composed of
stratified deposits are actually the result of
deposition of sediment by glacial streams
(glaciofluvial) and not by the movement of the
ice itself. These features are—
Outwash plains.
Eskers.
Kames.
Kame terraces.
Glacial lake deposits.
They result when the material in the glacier
has been carried and deposited by meltwater
from the glacier. The water selectively
deposits the coarsest materials, carrying
the fines away from the area. The end
result is essentially deposits of sands and
gravels.
Outwash plains result when melting ice at
the edge of the glacier creates a great volume
of water that flows through the end moraine
as a number of streams rather than as a continuous sheet of water. Each of the streams
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builds an alluvial fan and each of the fans
joins together and forms a plain that slopes
gently away from the end moraine area. The
coarsest materiaI is deposited nearest the end
moraine, and the fines are deposited at
greater distances. Much of the prairie land in
the United States consists of outwash plains.
Drainage and trafficability in the outwash
plains are much better than in a ground
moraine; however, kettles can be formed in
Surficial Geology 3-20
outwash plains due to large masses of ice left
during the recession of the ice front. If the
kettles are numerous, the outwash area is
called a pitted plain (see Figure 3-22, page
3-24).
Eskers are winding ridges of irregularly
stratified sands and gravels that are found
within the area of the ground moraine. The
ridges are usually several miles long but are
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rarelv more than 45 to 60 feet wide or more
than 150 feet high. They are formed by water
that flowed in tunnels or ice-walled gorges in
or beneath the ice. They branch and wind like
stream valleys but are not like ordinary valleys in that they may cross normal drainage
patterns at an angle, and they may also pass
over hills (see Figures 3-22, page 3-24, and
Figure 3-24, page 3-26). Figure 3-24, page
3-26, shows kettle lakes, swamps, and eskers.
A similar feature that resembles an esker,
but is rarely more than a mile in length, is a
ridge known as a crevasse filling. A crevasse
is a large, deep crevice or fissure on the surface of a glacier. Unsorted debris washes into
the crevasse, and when the surrounding ice
melts, a ridge containing a considerable
amount of fines is left standing.
Kames are conical hills of sands and
gravels deposited by heavily laden glacial
streams that flowed on top of or off of the
glacier. They are usually isolated hills that
are associated with the end or recessional
moraine; kettle lakes are commonly found in
the same area. The formation of kames normally occurred when meltwater streams
deposited relatively coarse materials in the
form of a glacioalluvial fan at the edge of the
ice; the fine particles were washed away.
This material accumulated along the side of
the ice, and when the ice receded, the
material slumped back on the side formerly in
contact with the glacier.
Delta kames are another type of kame that
may be formed when the meltwater flows into
a marginal lake and forms a delta. After the
lake and the ice disappear, deltas are left as
flat-topped, steep-sided hills of well-sorted
sands and gravels (see Figure 3-22, page
3-24).
Kame terraces are features associated with
alpine glaciation. When the ice moves down a
valley, it is in contact with the sides of the valley. As the glacier melts away from the valley
wall, glacial water flows into the space
created between the side of the glacier and the
valley wall. The void is filled with gravels and
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sands, while the fines are carried away by the
stream water. A terrace is left where the ice
was in contact with the valley; gravels and
sands can be found at the base of the terrace
(see Figure 3-25).
Glacial lake deposits occur during the melting of the glacier when many lakes and ponds
are created by the meltwater in the outwash
areas. The streams that fed these waters
were laden with glacial material. Most of the
gravels and sands that were not deposited
before reaching the lake accumulated as a
delta (later to be called a delta kame) after
melting of the ice. The fines that remained
suspended in the water were, on the other
hand, deposited throughout the lake. During
the summer, a band consisting of lightcolored, coarse silt was deposited, whereas a
thinner band of darker, finer-grained
material was deposited in the winter. The
two bands together represent a time span of
one year and are referred to as a varve.
Unstratified. Unstratified glacial deposits
(sediments deposited by the ice itself) are the
most common of the of glacial deposits. They
comprise the following surfical features:
Ground moraines.
End moraines.
Recessional moraines.
Drumlins.
Unstratified deposits make up landforms
that may be readily identified in the field, on
aerial photographs, and from topographic
and other maps. Unstratified deposits are
composed of a heterogeneous mixture of particle types and sizes ranging from clays to
boulders. Till is the name given to this mixture of materials. It is the most widespread of
all the forms of glacial debris. In general, features comprised of till are undesirable as
sources of military construction aggregate
since the material must be washed and
screened to provide proper gradation.
Ground moraines, sometimes called till
plains, are deposits that are laid down aS the
glacier recedes. Melting ice drops material
that blankets the area over which the glacier
traveled. A deposit of this kind forms gently
rolling plains. The deposit itself may be a
thin veneer of material lying on the bedrock,
or it may be hundreds of feet thick. Moraine
soil composition is complex and often indeterminate. This variation in sediment makeup
is due to the large variety of rocks and soil
picked up by the moving glacier (see Figure
3-22, page 3-24).
Surficial Geology 3-27
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Morainic areas have a highly irregular
drainage pattern because of the haphazard
arrangement of ridges and hills, although
older till plains tend to develop dendritic patterns. Frequent features associated with
grouncl moraines are kettle holes and
swamps. Kettles are usually formed by the
melting of ice that had been surrounded by or
embedded in the moraine material. Large
amounts of fines in the till prevent water from
percolating down through the soil. This may
allow for the accumulation of water in the kettle holes forming kettle lakes or, in low-lying
areas, swamps. Figure 3-22, page 3-24, and
Figure 3-24, page 3-26, show ground moraine
with an esker.
End moraines, sometimes called terminal
moraines, are ridges of till material that were
pushed to their locations at the limit of the
glacier’s advance by the forceful action of the
ice sheet. Generally, there is no one linear
element, such as a continuous ridge, evident
in either the field or on aerial photos. Normally. this deposit appears as a
discontinuous chain of elongated to oval hills.
These hills vary in height from tens to
hundreds of feet. The till material is. at
times. quite clayey. Kettle lakes are sometimes associated with terminal moraine
deposits also (see Figure 3-22, page 3-24).
Recessional moraines. which are similar to
end moraines. are produced when a receding
glacier halts its retreat for a considerable
period of time. The stationary action allows
for the accumulation of till material along the
glacier’s edge. A series of these moraines may
Surficial Geology 3-28
result during the retreat of a glacier (see Figure 3-22, page 3-24).
Drumlins are asymmetrical, streamlined
hills of gravel till deposited at the base of a
glacier and oriented in a direction parallel to
ice flow. The stoss side (the side from which
the ice flowed) of the drumlin is steeper and
blunter than the lee side. The overall appearance of a drumlin resembles an inverted
spoon if viewed from above. Drumlins commonly occur in groups of two or more.
Individual drumlins are seldom more than
½ mile long, and they can rise to heights of
75 to 100 feet (see Figure 3-26 and Figure
3-27).
It is important to understand that features
formed from the glacial process only occur in
certain areas of the world. Figure 3-28, page
3-30, and Figure 3-29, page 3-31, illustrate
the regions of the United States and the world
where glacial landforms occur. Table 3-3,
page 3-32, is a summary of glacial surficial
features.
EOLIAN PROCESS
In arid areas where water is scarce, wind
takes over as the main erosional agent. When
a strong wind passes over a soil, it carries
many particles of soil with it. The height and
distance the materials are transported is a
function of the particle size and the wind
velocity. The subsequent decrease in the
wind velocity gives rise to a set of wind-borne
deposits called eolian features.
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Types of Eolian Erosion
There are two types of wind erosion. They
are—
Deflation.
Abrasion.
Deflation. Deflation occurs when loose particles are lifted and removed by the wind.
This results in a lowering of the land surface
as materials are carried away. Unlike stream
erosion, in which downcutting is limited by a
“base level” (usually sea level), deflation can
continue lowering a land surface as long as it
has loose material to carry away. Deflation
may be terminated if the land surface is cut
down to the water table (moist soil is not carried away as easily) or if vegetation is
sufficient to hold the soil in place. In addition, deflation may be halted when the supply
of fine material has been depleted. This
makes a surface of gravel in the area where
deflation has taken place. This gravel surface
is known as desert pavement (see Figure 3-30,
page 3-32).
Surficial Geology 3-30
Abrasion. Abrasion occurs when hard particles are blown against a rock face causing
the rocks to break down. As fragments are
broken off, they are carried away by the wind.
This process can grind down and polish rock
surfaces. A rock fragment with facets that
have been cut in this way is called a ventifact
(see Figure 3-31, page 3-33).
Modes of Transportation
Soil particles can be carried by the wind in
the following ways:
Bed Load. Material that is too heavy to be
carried by the wind for great distances at a
time (mainly sand-sized particles) bounces
along the ground, rarely higher than two feet.
Suspended Load. These are fines (mostly
silts) that are easily carried by the wind.
Suspended loads extend to high altitudes
(sometimes thousands of feet) and can be
transported for thousands of miles. During a
particularly bad dust storm in the midwestern “dust bowl” on 20 March 1935, the
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Surficial Geology 3-31
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suspended load extended to altitudes of over
12,000 feet. The lowermost mile of the atmosphere was estimated to contain over 166,000
tons of suspended particles per cubic mile.
Enough material was transported to bring
temporary twilight to New York and New
England (over 2,000 miles away) on 21
March.
Eolian Features
Eolian surficial features may consist of
gravels, sands, or fines. The three main types
of eolian features areLag deposits or desert pavement.
Sand dunes.
Loess deposits.
Surficial Geology 3-32
Figure 3-32 illustrates the origin of these
deposits.
Lag Deposits or Desert Pavement. As the
wind billows across the ground, sands and
fines are continually removed. Eventually,
gravels and pebbles that are too large to be
carried by the wind cover the surface. These
remnants accumulate into a sheet that ultimately covers the finer-grained material
beneath and protects it from further deflation. Desert pavement usually develops
rapidly on alluvial fan and alluvial terrace
surfaces. The exposed surface of the gravels
may become coated with a black, glittery substance termed desert varnish. In some
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Transverse dunes are wavelike ridges that
are separated by troughs; they resemble sea
waves during a storm. These dunes, which
are oriented perpendicular to the prevailing
wind direction, occur in desert locations
where a great supply of sand is present over
the entire surface. A collection of transverse
dunes is known as a sand sea (see Figure
3-33a, page 3-34).
locations, the evaporation of water, brought
to the surface by the capillary action of the
soil, may leave behind a deposit of calcium
carbonate (caliche) or gypsum. It acts as a cement, hardening the pavement into a
conglomeratelike slab.
Although desert pavement contains good
gravel material, the layers are normally too
thin to supply the quantity required for construction. However, it does provide a rough
but very trafficable surface for all types of
vehicles and also provides excellent airfields.
Sand Dunes. Dunes may take several
forms, depending on the supply of sand, the
lay of the land, vegetation restrictions, and
the steady direction of the wind. Their
general expressions are as follows:
Transverse.
Longitudinal.
Barchan.
Parabolic.
Complex.
Longitudinal dunes have been elongated in
the direction of the prevailing winds. They
usually occur where strong winds blow across
areas of meager amounts of sand or where the
winds compete with the stabilizing effect of
grass or small shrubs (see Figure 3-33b, page
3-34).
Barchan dunes are the simplest and most
common of the dunes. A barchan is usually
crescent-shaped, and the windward side has a
gentle’ slope rising to a broad dome that cuts
off abruptly to the leeward side. Barchans
form in open areas where the direction of the
wind is fairly constant and the ground is flat
and unrestricted by vegetation and topography (see Figure 3-33c, page 3-34).
Parabolic, or U-shaped, dunes have tips
that point upwind. They typically form along
coastlines where the vegetation partially
covers the sand or behind a gap in an
obstructing ridge. Later, a parabolic dune
may detach itself from the site of formation
and migrate independently (see Figure 3-33d,
page 3-34).
Surficial Geology 3-33
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Complex dunes lack a distinct form and
develop where wind directions vary, sand is
abundant, and vegetation may interfere.
These can occur locally when other dune
types become overcrowded and overlap,
thereby losing their characteristic shapes in a
disorder of varying slopes (see Figure 3-33e).
Loess Deposits. In a number of regions of
the world, thick accumulations of yellowishbrown material composed primarily of
windblown silts make up a substantial
amount of surface area. These deposits are
known as loess. The material that makes up
these deposits originated mainly from dried
glacial outwash, floodplains, or desert area
fines. Loess is composed of’ physically ground
rock rather than of chemically weathered
material. The source and deposition point for
the material may be many miles apart, and
the deposits may range in thickness from a
few feet to hundreds of feet. Thickness tends
to decrease with distance from the source. In
the United States, most of Kansas, Nebraska,
Iowa, and Illinois are covered by loess. After
a loess has been laid down, it is rarely picked
up again. This is due to a very thin layer of
fines that interlock after wetting. While dry
loess is trafficable, it loses all strength with a
slight amount of water (see Figure 3-34).
Eolian features occur worldwide and may
consist of areas of sand dunes and desert
pavement or loess; however, their topographic expressions vary. In general, dune areas
are specified on maps by special topographic
symbols since they are continually changing unless stabilized by vegetation. Figure
3-35, page 3-36, is a topographic expression, using special symbols, of sand
Surficial Geology 3-35
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dunes and desert pavement (Summan). Figure 3-36 shows the generalized distribution
of eolian landforms throughout the world.
SOURCES OF CONSTRUCTION
AGGREGATE
Military engineers use their knowledge of
surficial features to develop borrow pits and
Surficial Geology 3-36
provide construction aggregate to meet mission requirements. Generally, engineer units
attempt to develop borrow pit operations in
fluvial features since they are easy to identify
and are normally accessible. In arid and
semiarid regions, eolian deposits and alluvial
fans provide large amounts of aggregate. In
mountainous regions and continentally
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Surficial Geology 3-37
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glaciated regions, fluvial-glacial deposits can
provide large quantities of quality aggregate.
Therefore, their presence should not necessarily be discounted in preference to fluvial
Surficial Geology 3-38
materials. Table 3-4 summarizes the types of
aggregates found in common fluvial, glacial,
and eolian surficial features.